The invention relates to a process for nylon depolymerization, in which process a multi-component material, comprising nylon and one or more non-nylon components, is fed to a depolymerization zone in which depolymerization of at least part of said nylon is effected, resulting in a product stream and a residue, said product stream containing monomers of said nylon, said residue containing non-nylon components.
Such a process is known from U.S. Pat. No. 5,681,952. In this publication a continuous process is described, where a multi-component, nylon 6 waste material and steam are fed continuously to a reactor. In the reactor depolymerization of nylon 6 takes place, and caprolactam may be recovered overhead, while a nylon 6 depleted bottom stream (residue) may be discharged from the bottoms. Since the composition of the feed material, in particular if the feed material is a multi-component waste material, is usually not constant, it is difficult to control the depolymerization process.
In view of the above it is an object of the present invention to provide an effective way to control a process for nylon depolymerization, in which process a multi-component material, comprising nylon and one or more non-nylon components, is fed to a depolymerization zone in which depolymerization of at least part of said nylon is effected, resulting in a product stream and a residue, said product stream containing monomers of said nylon, said residue containing non-nylon components.
According to the invention this object is achieved in that the nylon content in the residue is measured and used to control the depolymerization process. We have found that an efficient production of nylon monomers is possible according to the invention since the nylon content of the residue shows considerable variations with relatively small changes in depolymerization process conditions and with relatively small changes in the composition of the multi-component material which is fed to the depolymerization zone.
It is noted that WO-A-9749652 describes a process in which depolymerization of polyamides is effected in the presence of non-polymer contaminants. In the examples a batch process is described in which depolymerization of a glass-filled nylon 6,6, is effected, resulting in a product stream which contains monomers and in a residue, which contains glass fibers. In the examples the extent of the reaction is monitored by analyzing the product stream. The weight of the reactor residue is determined in order to calculate the polymer conversion. The nylon content in the residue is not used to control the depolymerization process.
As used herein, xe2x80x9cmulti-component materialxe2x80x9d denotes materials or articles that include at least one type of nylon and at least one non-nylon component. By xe2x80x9cnon-nylon componentxe2x80x9d is meant any material, excluding nylons, nylon depolymerization products, depolymerization agents, stripping agents, or depolymerization catalysts. By xe2x80x9cdepolymerizing agentxe2x80x9d is meant a solid, liquid or gas that will react with the amide linkage of the nylon to break the linkage, thus lowering the molecular weight of the nylon. Examples of depolymerizing agents are water, steam, alcohols, ammonia, amines. By xe2x80x9cstripping agentxe2x80x9d is meant a material which is in the gas phase at reaction temperature and which is able to carry away volatile reaction products of the depolymerization, e.g. monomers. The stripping agent may be the depolymerizing agent itself, such as for instance steam, gaseous ammonia or gaseous alcohol, or an inert gas, such as for instance nitrogen. By xe2x80x9cdepolymerizing catalystxe2x80x9d is meant a solid, liquid or gaseous material which catalyzes the breaking of amide bonds, such as for instance phosphoric acid, boric acid, phosphate salts, alkali metal oxides and alkali metal hydroxides. By xe2x80x9cnylon depolymerization productsxe2x80x9d is meant monomers or oligomers of nylon. The non-nylon components of the xe2x80x9cmulti-component materialxe2x80x9d may for instance be non-hydrolyzable polymers, such as for instance polyethylene, polypropylene, polystyrene, and copolymers thereof with butadiene, inorganic or organic materials, such as for instance fillers, pigments, dyes and/or other additives, or other types of materials. The non-nylon components may for instance constitute from about 2 to about 98, preferably from about 5 to about 95, more preferably from about 10 to about 90, and most preferably from about 20 to about 80 weight percent of the multi-component material which is fed to the depolymerization zone. These weight percentages are given with respect to the total weight of the nylon plus the non-nylon components in the multi-component material. The nylon may be any type of nylon, for instance nylon-6, nylon 6,6, nylon 4,6 as well as mixtures thereof. The nylon may for instance constitute from about 2 to about 98, preferably from about 5 to about 95, more preferably from about 10 to about 90, and most preferably from about 20 to about 80 weight percent of the multi-component material which is fed to the depolymerization zone. These weight percentages are given with respect to the total weight of the nylon plus the non-nylon components in the multi-component material. Preferably, at least 40 wt. %, more preferably at least 50 wt. %, in particular at least 75 wt. % and more in particular at least 90 wt. % of the total amount of nylon in the multi-component material is one type of nylon. Preferably, at least 40 wt. %, more preferably at least 50 wt. %, in particular at least 75 wt. % and more in particular at least 90 wt. % of the total amount of nylon in the multi-component material is nylon 6.
The nylon content of the residue may be measured using any technique that allows the determination of the nylon content in the residue. The nylon content in the residue may be expressed by any characteristic from which the amount of nylon in the residue relative to the amount of other components in the residue, preferably relative to the amount of non-nylon components in the residue can be derived. Examples of suitable measurement techniques are high performance liquid chromatography, density measurements, Raman spectroscopy and near infrared spectroscopy. Preferably, measurements techniques are used in which the analysis time is not too long, preferably less than 1 hour. Most preferably, near-infrared spectroscopy is used. Using this technique a quick and acurate measurement of the nylon content is possible.
According to the invention the content of a specific type of nylon in the residue may be determined, for instance the nylon 6 or the nylon 6,6 content. According to the invention it is also possible to measure the total content of all nylons in the residue.
As used herein, xe2x80x9cmeasuring the nylon content of the residuexe2x80x9d, denotes the determination of the nylon content of the residue during the depolymerization process, either continuously or at intervals. If the nylon content is determined at intervals, it is preferred that frequency of the measurements of the nylon content is sufficiently high that adjustments to the process parameters may be made on time. In order to be able to make adjustments to the process parameters as quickly as possible, it is desired that the time, required for the determination of the nylon content is as short as possible. If the process is very stable, the time interval between two measurements may be longer than if the process is less stable.
The value of the nylon content, e.g. expressed in weight percent nylon, is used to control the depolymerization process. Preferably, the control of the depolymerization process is carried out by comparing the measured nylon content in the residue with a desired value, and adjusting at least one process variable in accordance with the difference between the nylon content in the residue and the desired value. Examples of such process variables are the temperature at which depolymerization is conducted, the rate at which the multi-component material is fed to the depolymerization zone, the feed rate of steam, water alcohol, ammonia and/or amines to the depolymerization zone, the reactor pressure, and/or residence time, but the invention is not limited thereto. The conversion of nylon-6 to caprolactam may for instance be enhanced by increase of steam flow, increase of steam temperature, increase of the temperature at which the depolymerization is conducted, and increase of residence time in the depolymerization zone, which may for instance be achieved by a decrease of the rate at which the multi-component material is fed to the depolymerization zone. If the nylon content in the residue is found to differ too much from the desired value, at least one process variable may be adjusted in such a way that the nylon content in the residue obtains the desired value again. These adjustments may be made manually by an operator, but it is also possible to have the adjustments made in an fully automated way.
The process according to the invention can be carried out batchwise or continuously. The process according to the invention is preferably carried out continuously, since the control of the depolymerization is than particularly effective.
Advantageously, the nylon content in the residue is determined using near infrared spectroscopy. We have found that this technique allows a quick and accurate measurement of the nylon content in the residue, even if the nylon content in the residue is low.
A description of near infrared spectroscopy the application of this technique can for instance be found in manuals of commercial infrared spectrometers. Any type of near infrared spectrometer may be used. A dispersive near infrared spectrometer is preferred.
Typically, a predictive model is created for the value of the nylon content in the residue. Said predictive model gives a correlation between the nylon content of samples of the residue and the near infrared spectra of said samples. In order to create such a predictive model, near infrared spectra of residue samples having various nylon contents may be measured, the nylon contents of said samples having been measured by independent means, for instance by HPLC. Preferably, a spectral pretreatment is applied to the near infrared spectra to obtain, for instance, first or second derivatives of the raw data. However, other spectral pretreatments are also possible. Based on these data, the predictive model may then be calculated using well-known mathematical techniques which, for instance, include Multi-Linear Regression (MLR) and Partial Least Square (PLS) models. However, other mathematical techniques are also possible. For these calculations, commercial software may be used which is often supplied by the supplier of the spectrometer. In order to measure the nylon content of the residue during the process, near infrared spectra of the residue may be obtained. The nylon content of the residue may then be calculated using the predictive model. These calculations are preferably carried out using a computer which is connected to the spectrometer. Preferably, the near infrared spectra of the residue are subjected to a spectral pretreatment to obtain, for instance, first or second derivatives of the raw data.
We have found that the color of the residue may differ for different feed materials. Advantageously, the near infrared spectroscopy is performed using different predictive models for different colors of the residue. In this embodiment different predictive models are created for different colors of the residue. The nylon content of the residue may then be determined by measuring the near infrared spectra, and by calculating the nylon content using a predictive model, said predictive model being selected depending on the color of the residue. Said selection may be done in a fully automated way since the color may usually be derived from the near infrared spectra. The use of different predictive models for different colors of the residue improves the accuracy of the measurements of the nylon content of the residue, if the color of the residue varies. Typical colors of the residue are brown or black, though other colors are possible.
Preferably, near infrared spectra are obtained in the range between 1100-2500 nm. For the determination of the nylon 6 and nylon 6,6 content in the residue, preferably spectral information in the range of 2000 to 2200 nm is used, since in this range the interference with spectra of non-nylon components, such as for instance Styrene-Butadiene Rubber and/or polypropylene is relatively small.
The residue may be available as a continuous molten stream, since the depolymerization is usually conducted at elevated temperatures. The residue may also be available in the solid state. Such a residue may for instance be obtained by cooling and solidifying the molten stream. In a preferred embodiment the near infrared spectroscopy is performed on residue which is molten. This embodiment is in particular advantageous if the measurements of the nylon content in the residue are carried out continuously. In another preferred embodiment the near infrared spectroscopy is performed on residue which is in the solid state, e.g. in flake form. Preferably, the near infrared spectroscopy is performed on residue which is a granulate or powder, the particle diameter preferably being below 1 mm. Such a granulate or powder may for instance be obtained by milling the flakes. This gives a more accurate result, since small inhomogenities are averaged during the milling operation.
Preferably, the nylon content of the multi-component material which is fed to the depolymerization zone is measured, and used to control the depolymerization process. If the data for the nylon content in the multi-component material which is fed to the depolymerization zone and the nylon content in the residue are combined, a very accurate control of the depolymerization is possible, especially when the composition of the feed is not constant. In practice, the nylon content of the feed may vary, if the feed is waste material and if different sources and/or suppliers of the waste material are used. For instance, if the waste material is beneficiated carpet, its nylon content is likely to be dependent on the beneficiation process. The measurement of the nylon content in the multi-component material is not limited to a specific method. Preferably, near infrared spectroscopy is used.
As a result of the depolymerization in the depolymerization zone, a product stream and a residue are obtained.
The product stream contains monomers of the nylon obtained by the depolymerization. Monomers of nylon 6 include xcex5-caprolactam. Monomers of nylon 6,6 include hexamethylenediamine, aminocapronitrile and adiponitrile. Typically, the product stream includes as a major constituent monomers of the nylon. By xe2x80x9cmajor constituentxe2x80x9d it is meant that the monomers are the largest constituent of the product stream by weight, excluding depolymerizing agents, stripping agents and depolymerizing catalysts.
The residue contains at least one non-nylon component. By xe2x80x9cnon-nylon componentxe2x80x9d is meant any material, excluding nylons, nylon depolymerization products, depolymerization agents, stripping agents, or depolymerization catalysts. Typically, the residue contains residual non-nylon components. By xe2x80x9cresidual non-nylon componentsxe2x80x9d is meant materials which originate from the non-nylon components in the multi-component material, and which remain after exposure to the conditions at which depolymerization of the nylon is effected. Typically, the residue is depleted of nylon. By xe2x80x9cdepleted of nylonxe2x80x9d is meant that the weight percentage of nylon in the residue is smaller than that in the multi-component material which is fed to the depolymerization zone. Said weight percentages denote the weight percentage of nylon in respect of the total weight of the nylon plus non-nylon components.
In general, it is desirable that the nylon content in the residue is as low as possible, since a lower value of the nylon content in the residue corresponds to a higher conversion of nylon into nylon monomers for a constant flow and contents of the multi-component material. In that case the process parameters may be adjusted, if the nylon content in the residue is found to exceed a certain value. However, it will be clear that it is also possible that the nylon content in the residue is allowed to vary between a certain maximum and minimum value. The desired value of nylon content of the residue will usually be in the range of 0-30 wt. %, preferably from 0.5-20 wt. % and more preferably in the range of 1-10 wt. %. These percentages are given as the weight percentage of the nylon in the residue with respect of the total weight of the nylon plus non-nylon components in the residue.
The depolymerization is not limited to a specific method. Usually, the depolymerization is conducted at a temperature of more than 250xc2x0 C., preferably more than 280xc2x0 C., more preferably more than 300xc2x0 C. Typically, the depolymerizaton is conducted at a temperature of below 400xc2x0 C., preferably below 350xc2x0 C., more preferably below 340xc2x0 C. In a preferred embodiment the depolymerization is carried out in the range of 280-340xc2x0 C. The depolymerization may be carried out at reduced, atmospheric or increased pressure. Preferably, the depolymerization is carried out in the presence of a depolymerizing agent, such as for instance water, steam, alcohol, ammonia, or amines. Most preferably, the depolymerization is carried out in the presence of water or steam. The depolymerization may be effected in the presence or in the absence of a catalyst. Any type of reactor in which nylon may be depolymerized may be used, such as for instance a continuous stirred tank reactor, a tubular reactor or a series of reactors.
In the depolymerization zone the depolymerization may be effected in many ways. Examples of suitable processes have been described in U.S. Pat. No. 5,681,952, EP-A-737666, WO-A-9749652, WO-A-9618612, WO-A-9406763, WO-A-9706137, WO-A-9618613. In the depolymerization zone, depolymerization may for instance be effected resulting in a vapor stream containing monomers of the nylon and in a residue. The multi-component material may for instance be contacted with a vapor, for instance steam, gaseous ammonia, or gaseous alcohol, resulting in a vapor stream containing monomers of the nylon and in a residue. The nylon content of said residue may then be measured and used to control the depolymerization according to the invention. In another embodiment a mixture comprising water and the multi-component material is subjected to an elevated temperature and pressure to form a liquid aqueous solution which includes monomers of the nylon and a water insoluble portion, and the liquid aqueous solution and the water insoluble portion are separated. Such a process has for instance been described WO-A-9706137, WO-A-9618613. The nylon content of said residue, i.e. the water insoluble portion, may then be measured and used to control the depolymerization according to the invention.
The process according to the invention is in particular suitable if the multi-component material includes nylon 6 and if the depolymerization is conducted in the absence of added catalyst with superheated steam at a temperature of about 250 to 400xc2x0 C., preferably at a pressure within the range of about 0.1 MPa to about 10 MPa and substantially less than the saturated vapor pressure of water at said temperature. As a result a vapor stream which contains caprolactam is obtained. Said process has been described in detail in U.S. Pat. No. 5,681,952. The nylon content of said residue may then be measured and used to control the depolymerization according to the invention.
Preferably, the multi-component material is nylon-containing waste material or nylon-containing processed waste material, waste material meaning material or articles which has been, is intended to be, or would have been discarded by a consumer, manufacturer, distributor retailer, installer and the like. Preferably, the waste material is waste carpet material, and more preferably waste carpet material that includes nylon 6 face fibre and non-nylon components. Carpets include a face fibre that is adhered to a backing (support) material which may include jute, polypropylene, latex (such as styrene-butadiene rubber (SBR)) and a variety of inorganic materials such as calcium carbonate, clay or hydrated alumina fillers. As used herein, xe2x80x9ccarpet materialxe2x80x9d denotes carpet which has not been subjected to any mechanical separation (referred to herein as xe2x80x9cwhole carpetxe2x80x9d), as well as any mixture of carpet components that is a product of separation, mechanical or otherwise, of whole carpet (referred to herein as xe2x80x9cbeneficiated carpetxe2x80x9d). xe2x80x9cWaste carpet materialxe2x80x9d denotes carpet material that has been, is intended to be, or otherwise would have been discarded by a consumer, manufacturer, distributor, retailer, installer and the like.